Source-to-drain punch-through is a condition that occurs when the depletion region of the drain reaches the source, shorting the source and drain regions of the device. One method of avoiding this condition is to raise the doping level beneath the channel between the source and drain regions of the transistor. The raised doping region is typically formed by implanting between the source and drain regions a complementary dopant to the dopant of the source and drain regions to form a punch-through implant. For example, if the source and drain are doped with n-type dopants, the punch-through implant is doped with a p-type dopant. Higher doping reduces the extent of the drain depletion region, preventing the occurrence of source-to-drain punch-through. A problem with typical methods of implanting the punch-through implant is that it is difficult to control the precise location of the dopant and the resulting extent of the implant region. The edges of the implant region often overlap the source and drain regions of the transistor. This overlap creates a disadvantage of increased junction capacitance in the device.
The concept of a highly doped implant region is also useful in the formation of back gate transistors and back body contact transistors. These devices utilize polysilicon back gates and back body contacts, referred to generally herein as back contacts, disposed within a buried oxide layer of the transistor. In these devices, it is desirable to implant an area of dopant into the back contact to reduce the contact's resistivity. As with the punch-through implant, however, typical implantation techniques make it difficult to precisely control the area of implantation. In the back body contact transistor, stray implantation results in increased junction capacitance. In the back gate transistor, stray implantation results in increased MOS overlap capacitance.